2. All The Action Revolves Around Another Set Of Characters That Have Their Own World, Rules, And Politics
Most shows spin-off when the interplay between a fringe group or character within the show starts to take over the plot. Look at shows like The Originals, Angel, or even Private Practice. Not Supernatural. Supernatural used to focus on a ghost or demon of the week and how their intrusion on the regular world causes chaos in our society. Per episode, the human to supernatural being ratio was tilted heavily to the human side. Now Supernatural has expanded to focus on the politics of heaven, hell and purgatory and how humans (mostly Sam and Dean) causes chaos in their society. Per episode the supernatural beings vastly out number the humans. This isn't a new development. In fact, the focus on divine politics has lasted seasons longer than the shows original monstrous cast. Every now and then the writers treat fans to a nostalgic episode like "Back to School Special." Those episodes make fun, nostalgic, heart-warming breaks. Fans of the show enjoy these episodes even as a sense of dread overwhelms them because by now the pattern is clear. Nostalgia this week means something very bad is coming in the next episode of Supernatural.